1. Field of the Invention
The present invention pertains to the field of mass flow metering devices, such as Coriolis flow meters. More specifically, the metering devices are used in combination with other devices or estimation techniques that determine the composition of fluid in a flow stream by mass fraction on a real-time basis to gain superior measurement accuracy.
2. Statement of the Problem
Industrial processes that consume or transport petrochemical liquids and gasses often use a mixture of compounds, e.g., methane, ethane, propane, and butane in a single mixture. It is often important to know the percentage of the total mixture that consists of a single type of compound. In this context, representation of the compound in the mixture is often discussed in terms of a mole fraction or a mass fraction. The term xe2x80x9cmass fractionxe2x80x9d means a percentage of a mixture allocated to a single compound or group of compounds on a mass basis. Similarly, the term xe2x80x9cmole fractionxe2x80x9d means a percentage of a mixture allocated to a single compound or group of compounds on a mole basis. It has been a conventional practice to calculate mole or mass fractions based upon volumetric measurements of the combined flow stream that are converted to mass using various empirical correlations or density measurements. This conversion process adds uncertainty and error into the mass fraction determination.
A specific example where it is desirable to ascertain mass fractions or mole fractions from a flow stream exists in the petrochemical refining industry. Engineers are constantly reviewing process efficiencies in the cracking of various feedstocks to convert these feedstocks into refined products, e.g., as in the flame-cracking process of making ethylene directly from petroleum by combustion at 2000xc2x0 F. using a mixture of napthalene or crude oil and high temperature gasses with support from pure oxygen. Depending upon the nature of the crude oil and the availability of gasses, the reaction temperature and timing may be adjusted to optimize the economic recovery from use of the reactor vessel. Mass balance calculations based upon compositional fractions in the flow stream are often essential to these general types of calculations. In this context of process fine-tuning adjustments, it is not only useful to know the percentage composition of the incoming fuel stream, but it is also useful to know the percentage composition of the reaction products. These measurements are typically performed on volumetric percentages, as opposed to mass percentages.
Another specific instance where it is desirable to know the fractional breakdown of a flow stream exists in the use of pipelines for transportation and delivery of natural gas and other fuels. Fuels are typically sold on a volumetric basis, but the heating value may vary by more than fifty percent on a constant volumetric basis depending upon variation of the fuel composition over time.
Yet another example of a need for mass fraction analysis exists in instances where neither mass nor volume are measured. For example, an internal combustion engine or an industrial boiler may be operated for the express purpose of burning fuels to produce electricity. The engine is used to turn a small generator for this purpose. The boiler may be used to make steam that drives a larger generator. While the ultimate goal is to harness energy from these fuels, the energy throughput into the engine is not measured.
It is substantially impossible to perform a direct or indirect measurement of chemically available energy that resides in a feedstock based upon an analysis of work output and system energy losses. The act of combustion is associated with an efficiency loss, e.g., 40% to 60%, in which a portion of the chemically available energy stored in the feedstock is lost to entropy because it cannot be converted into useful work. For example, heat is lost by convection and radiative transfer. Exhaust gasses are hotter due to the exothermic nature of combustion. Fuels almost never have consistent quality. These factors combine to prevent the monitoring of combustion efficiency as an indicator of efficiency or impending mechanical failure in a mechanical device.
Boilers and engines may be adapted to use different fuels. For example, a dual-purpose boiler may be easily converted from use with a gas feedstock to use with a liquid feedstock. This type of switching boiler has application in the public service company sector where an electric company may wish to switch between fuels to minimize its expenditures for fuel or to reduce levels of regulated emissions. It can be very difficult to switch a boiler from oil to gas and then to ascertain how much gas must be consumed to replace the oil when the nature and content of the gas fuel is unknown.
Even where the change in feedstock is not so drastic as a switch from oil to natural gas, the feedstocks themselves vary in quality and composition over time. Diluents including carbon dioxide, nitrogen, water, and hydrogen sulfide are commonly found in natural gas flow streams. Furthermore, the relative percentages of constituents in natural gas have large variations by producing region, in addition to variations from well to well in a selected producing region. Thus, gas that is produced from the Gulf of Mexico region may have a lower specific gravity and energy content than gas that is produced in Nigeria or California. Similarly, the nature of crude oil varies from tar-like substances to thinner oils that pour easily and have a light brown color. In transportation, individual flow streams are mixed and combined as the materials are transported by pipeline or by ship from the producing regions to the consuming regions. Each flow stream has its own composition and specific heating value.
An engine or boiler operates at a different efficiency depending upon the nature and quality of the fuel that it burns. Even where an engine rotates at a constant speed, a change in feedstock constituents by the addition of diluents may make less torque available from the engine. Similarly, a boiler may make less steam. The combustion devices may suffer a diminution in or improvement in efficiency if a natural gas supply changes to one having relatively more methane. If only the heating content of the fuel were known, it would be possible to alter the operating conditions of a combustion device according to a preselected parameter, such as a change in volumetric or mass flow rate to provide a constant energy source or operation of the device within a preferred range for obtaining an optimal fuel efficiency.
As reported in Snell et al., Installation of Multipath Ultrasonic Meters on a Major Australian Metering System Project, in December of 1996, multipath ultrasonic flow meters (volumetric meters) were installed on an Australian natural gas pipeline for use as custody transfer meters at all offtakes from the transmission system into local distribution systems. The meters were each coupled with a gas chromatograph that analyzed the flow stream constituents. The mass flow measurements were converted to volume and volume-based enthalpy values were calculated for the flow stream. Ultrasonic meters were chosen for the study because they were reported to have the least uncertainty in measurement for both volume and energy content for the flow rates in the study. Coriolis meters were listed as possible alternatives to obtain volumetric flow measurements using AGA equations to convert the mass flow readings into volume, but Coriolis meters were also characterized as having the greatest uncertainty in energy measurements, i.e., 3.0% versus 1.0% for ultrasonic meters. All of the energy uncertainties for all types of meters in the study were presented as being greater than the volume uncertainties.
As shown in the above discussion, a mass-based metering device that could provide an accurate real time determination of the mass fractions in a flow stream would facilitate mass balance calculations in petrochemical refining, as well as open new horizons in the ability to conserve and sell energy values with decreased levels.
The present invention overcomes the problems outlined above by providing a metering device that affords extremely accurate direct measurements concerning the mass fractions of a flow stream. These mass fractions may then be related to mass balance calculations in petrochemical refineries, as well as heat content or other enthalpy-related values that are available from a flow stream. Accuracy is improved by avoiding the former necessity of converting mass based flow measurements to volumetric measurements as a condition precedent to ascertaining mass fractions.
The metering device is used to provide real time telemetry concerning mass fractions in a flow stream having multiple constituents. A Coriolis mass flow meter or other mass-based flow meter is used to measure a mass flow rate in the flow stream and to provide first signals representative of the mass flow rate. A chromatograph, density or pressure measurement combined with an empirical correlation, or other means for analyzing the content of the flow stream is used to determine constituent percentages of the flow stream and to provide second signals representative of the constituent percentages.
In preferred embodiments, a central processor, computer or controller is used to interpret the first signals and the second signals received from the analyzing means to provide output representing an energy value in the flow stream. The energy value derived by multiplication of the mass flow rate times the constituent percentages times mass-based energy values of constituents corresponding to the constituent percentages. This computational technique is advantageous because it permits a direct or mass-based computation of energy content in the flow stream while minimizing intermediate correlations, such as correlations approximating the nonideal behavior of real gasses.
In still other preferred embodiments, the energy metering device is operably coupled with a throttle to control flow based upon energy content of the flow stream based upon a preselected parameter. According to principles of the invention, the preselected parameter may include delivery of a substantially constant rate of energy for release by combustion, delivery of energy at a rate within a preferred operating range for a combustion device, or delivery of time-controlled sales of energy content in the flow stream.
When the mass flow meter is a Coriolis meter, the meter can also be operated as a densitometer, and the density readings can be used to associate the flow stream with an empirical correlation of energy content, as an alternative to use of a chromatograph to analyze constituent percentages.